Method of metal preparation for porcelain enameling



Nov. 22, 1960 BRYANT ET AL 2,961,337

METHOD OF METAL PREPARATION FOR PORCELAIN ENAMELING Filed Oct. 8. 1958 STEEL WORK PIECE 3 TO BE ENAMELED i BOILING ALKALINE CLEANER (2% NAIO) DRAIN RINSE CLEAN RUNNING CLEAN wATEIa OVER --4------ wATEIz |70-2IOF HEATING coILs DRAW RINSE CLEAN RUNNING CLEAN WATER WATER 200M TEMP. OR LESS DRYER-CLEAN CIIZCULATING ATMOSPHERE 300 -375? 2 M INUTE SOAK PIECE COOLED To zoom TEMPERATURE UPON REMOVAL FROM FURNACE 6 MINUTES IN ACID PlCKl-E REMOVAL OF PHOSPORIC ACID DEAIN RINse- CLEAN kUNNllaG wA-I-EIa OF CLEAN WATER AT o2 lGH LY ACID OF 5.0 To

35 F oM AcIo dim ovse BELOW ROOM TEMPRATUEE 4.010.25 oz/GAL.

(a 0* 2 F ugh CLEAN WATER oven I60 0 OR K muons HEATING come.

I DRYER CLEAN CIIZCULATING ATMOSPHERE an 300- 4oc F PORCELAIN ENAMEL (Oven COAT mlitron m INVENTOR.

Z/yene 5. Bryan! BY fly/M oNIz COAT ENAMELED W fi SHEET swam. woI2I PIECE 1 M m ArraEA/Ens' United States Patent .METHOD OF METAL PREPARATION FOR PORCELAIN ENAMELING Eugene E. Bryant, Cleveland, and Vaylord R. Luster,

Medina, Ohio, assignors to Ferro Corporation, Cleveland, Ohio, a corporation of Ohio Filed Oct. 8, 1958, Ser. No. 765,959

16 Claims. (Cl. 117-50) herence of the enamel and for uniform deposition of nickel in step three (3) which follows. The acid etch hasalso been effected by substituting an abrasivesystem for the acid treatment consisting generally of a sand or shot blasting operation. (3) A nickel flash or deposition either by an ion exchange method or electroless nickel deposition whereby a thin coating of metallic nickel is deposited, or plated onto the relatively clean and etched work piece, in an acid solution of pH in the range of 3.0 to 6.0. (4) Neutralizer bath of alkaline solution to prevent further oxidation caused-by acidic residues and (5) drying. 'The importance of the nickel deposition step noted above is well-known.

Utilizing the above noted sequence of steps, slight traces of organic greases and foreign matter acquired in milling ofthe-steel, in transit, and fabricatingthe piece were not completely removed by the cleaning methods previously used, nor in subsequent acidtreatment operations, so that such retained traces of organic matter, when subjected to the-h1gh-temperatures of-fusion of'the vitreous enamel cover coats in the range of 1350 to 1500-." F., volatilize and tend to give rise to pits and blisters in the direct-on cover coat. These pits and blisters form surfaceblemishes in the finished piece. It being clearly understood by one skilled "in the art of course that the usecf less viscous and more-flexibleintermediate ground coats prior to the application of the cover coat would eliminate the problem of defects, but the use of such intermediate ground coat is undesirable because of the need for an additional application, drying and firing operation plus the additional material required.

The natureof the surface ofthe steel and ferrous allQYS isimportant in its preparation for direct-on application of cover coats. The surface should be uniform for good results. ,However, the surface of said ferrous alloys variesfrom millto mill and from lot to lot produced in thesame millsufficiently to cause a variation in the rate .Qfattack of acid pickling solutions resulting in a consequential variation in the degree of etch effected by the acid resulting in a consequential variation in the rate of ,depositionof nickel per unit area to the steel surface so that the criticallimits of nickel deposition for a given operation tend to vary sufiiciently to adversely affect the surface and adherence qualities of the, finished. enameled coating. This results in time-eonsurning delayswhile. the

temperature, pH and length of time of immersioncf the "ice work piece in the acid and nickel solutions arereadjusted to each lot of steel to compensate for variations insurface structure, in order to achieve a consistent degree-of etch and deposition of nickel on the finished-work piece prior to enameling.

Thus we have found that by utilizing a physical heat treatment step in conjunction with any of the previously used steel preparation methods the variable skin of the ferrous alloy can be quickly andeffectivelyremoved. Furthermore, all minute traces of foreign material usually insoluble to some extent in either the preceding cleaning or subsequent etching steps are removed by oxidizing both the skin plus traces of foreign matter so they can be uniformly and completely removed by subsequent etch treatment.

In employing the heat treatment step, we have found that temperatures less than 1000 F. are incapable of producing the desired degree of oxidation, and temperatures above 1300" F. result in excessive and uneven oxidation.

For the most uniform removal of the skin and foreign matter from ferrous metal surfaces, temperatures inthe range of 1100 F. toabout 1200 F. are used, oxidizing the steel or ferrous alloy work piece the equivalent of from one to four grams of the ferrous base metal per squarerfoot.

It is an object of this invention to provide a method of preparing a ferrous work piece for a direct-on application of cover coat which is free of surface blemishes.

It is an object of this invention to provide a new method ofpreparation of ferrous base metal for the direct application of vitreous cover coats in which the effect of work solutions on variable ferrous base metals is stabilized and an improved surface is produced for the application of direct-0n vitreous enamel.

Itis further the object of this invention to provide a method of porcelain enameling an article characterized by the elimination of conventional, intermediate ground coats in favor of a single, blemish-free finish coat applied directly on the base metal rather than over said intermediate coat.

It is a further object of this invention to provide a method for producing a superior enameled article by the application of a single finish coat direct to steel by depositing a highly uniform and even coating of nickel on its surface prior to application of the enamel cover coat.

In carrying out the invention, it is important that the oxidizing heat treatment be performed before the etching step.

While the use of a grease burn for the sole purpose of removal of mill oil and shop lubricants from theferrous work piece has been known for some time, such (grease burn was in lieu of the cleaning operation-rather than supplementary to it, and was performed at the elevated temperatures of 1500 to 1600" F. Furthermore,

such heat treatment, in lieu of a chemical or abrasive cleaning operation, had the effect of producing highly variable, uneven, and excessive oxidation of the work piece, said oxidation consisting of a complex mixture of insoluble iron soaps, partially oxidized organic compounds and metalparticles, which were carried over into the Sub sequent acid and/or nickel application steps, resulting .in heavy contamination of these baths with resultant imperfections and blemishes in the finished cover coat, by virtueof the contaminants being picked up on thesurface of the work pieces subsequently passed through said baths. On the other hand, the present invention utilizing the combination of oxidizing heat treatment. at limited temperatures and an acid or abrasive etchproduces an article having a light, thin, tight oxideulayer containing only traces of oxidized organic matter unremoved by the preceding cleaning-operation, ahdconsisting principally of the purer forms of oxides of iron an evenly etched surface in the following etching step,

which evenly etched surface, in turn, permits the even and uniform application of nickel over said evenly etched planular surface, which even application is so vital to the successful production of blemish-free enamel cover coats applied directly to steel.

STEEL SELECTION While any grade of steel having less than 0.05% carbon generally may be utilized in conjunction with this invention, good results may be obtained by using a cold rolled steel, having the following metalloid analysis:

Percent Carbon 0.03 Manganese 0.05 Phosphorous 0.004 Sulphur 0.025 Silicon Trace The gauge of the metal is limited only by the design of the work piece and ease of forming, and does not in any way alter the application of our process, except that the oxidizing heat treatment must be such that the article itself reaches the temperature indicated in the furnace, and then be permitted to soak for a period of about two minutes at indicated temperature.

The design of the article is of no particular importance, and since any one of innumerable designs and shapes have been successfully enameled, the subject article will be referred to as work piece throughout the specification and claims.

The work piece in accordance with the present invention is formed to a pre-designed shape and after forming, is processed by the following steps:

Alkaline cleaning, heat treatment, cooling, acid or abrasive etching, rinsing, nickel deposition, rinsing, drying, cover coat application, drying and firing.

The following example, in conjunction with the accompanying flow sheet drawing, illustrates the present invention:

Example A 20 gage cold rolled steel sheet having an analysis as shown above, was formed into a panel and processed as indicated below.

I. Cleaning perati0n.The work piece was submerged in an alkaline cleaning bath at a rolling boil at 212 F. for a minimum of 20 minutes. The cleaner was maintained at an active alkali content of 2% Na O.

II. Rinsing.The work piece was removed from the .cleaner bath and submerged for from one to two minutes in clean, running water, maintained at a temperature of from 170 to 210 F.

III. Secondary rinsing-The work piece was then immersed in a cold, clean water rinse at room temperature or less, for two minutes.

IV. Drying-The alkaline cleaned and rinsed work piece was placed in a dryer, providing a cleancirculating atmosphere, at a temperature between 300 to 375 F. It is understood that any suitable drying means may be used including continuous gas fired, oil fired, electrically heated or infra red dryers providing the work pieces are kept free of dirt or other foreign matter.

V. Oxidation.-Thework piece was subjected to a temperature of 1200 F. for four minutes, two minutes being required to bring the piece to peak temperature, plus two additional minutes at the indicated temperature in a commercial furnace having an oxidizing atmosphere.

VI. C00ling.-The work piece was removed from the furnace and permitted to cool to room temperature.

VII. Acid pickle.-The oxidized work piece was submerged in a phosphoric acid bath of 17:2% by volume, at a temperature of F., for a time of six minutes, during which time all oxides were completely removed, and an even etch is achieved by the acid bath.

VIII. Rinse.-Work piece was submerged in clean running water of slightly acid pH (5.0 to 6.5) for a time of from IV: to 2 minutes, said clean, running water at room temperature or below.

IX. Nickel deposition-Using the well-known electroless method of, application, such as described in US. Patent 2,532,283, the work piece was immersed in a bath containing 4.0 ouncesi0.25 ounce per gallon of single nickel salts (Na SO .6H O); 1.0: /2 ounce sodium acetate per gallon; and 25 gramsiS grams sodium hypophosphite per gallon. This bath was maintained at a temperature of 87 E12 at a pH 35:03, for a time of 3 /2 minutes achieving a deposition of nickel by reduction on the work piece of approximately 0.065 gram of nickel per square foot of area exposed to the bath.

X. Rinsing.-The nickel-coated work piece was rinsed in clean, clear running water maintained at a temperature of F.i20 F. for a time of 2 minutesi /i minute.

XI. Drying-The work piece was placed in a suitable dryer having a clean, circulating atmosphere at a temperature of 300 F. to 400 F.

XII. Enameling.-After permitting to cool to room temperature, after drying, a titanium opacified cover coat of the composition listed below was applied so as to give a fired thickness of 0.005 inchi0.002 inch.

FRIT COMPOSITION After the enamel is applied to the work piece, it is dried and fired in an enameling furnace at a temperature 1400 F. and for a time in the hot zone of the furnace to maintain the work piece at the indicated temperature for a period of 1% minutes, for a total time of four minutes.

While the example set forth describes submersion, of the work piece in various baths, this is meant to convey the idea of exposure of the work piece to the various baths described, either by actual immersion in a tank or vat containing the various solutions, or exposure of the work piece to the various solutions through a continuous spray machine wherein the solutions are maintained at the indicated temperatures and concentrations and the spray areas are of a length suflicient to subject the work piece to a given solution applied under pressurized spray for the required length of time.

It should be here pointed out also, that, although we have used phosphoric acidand electroless'nickel in-the example, it is well known to those skilled in the art that any number of acids, such as hydrochloric and sulphuric, which will oxidize and etch steel will perform satisfactorily, as well as any method of applying the nickelcoating which includes deposition'by ion exchange and by re duction.

It is to be understood of course that variations may be made on this invention in order to meet different requirements without departing'from the scope and intent of these specifications and the claims which follow.

Having thus described the invention, we claim:

1. In the-processof preparing ferrous base metal for direct-on porcelain enameling in-which-the metal is subjected to a preliminary cleaning operation, the steps following said cleaning operation of heat-treating the ferrous base metal at a temperature of between about 1000 F. and 1300 F. long enough tooxidize from about'one to four grams base metalper square foot of surface, and thereafter applying -a porcelain enamel cover coat-directly to the base metal.

2. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface and thereafter etching with phosphoric acid, neutralizing, drying and applying a finish coat of porcelain enamel directly to the metal.

3. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface and thereafter etching with sulphuric acid, neutralizing, drying, applying a cover coat of porcelain enamel directly to the metal and fusing the porcelain enamel thereon.

4. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface and thereafter etching with nitric acid, neutralizing, drying, applying a cover coat of porcelain enamel directly to the metal and fusing the procelain enamel thereon.

5. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000" F. and 1300" F. long enough to oxidize from about one to four grams base metal per square foot of surface thereafter etching with hydrochloric acid, neutralizing, drying, applying a cover coat of porcelain enamel directly to the metal and fusing the porcelain enamel thereon.

6. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by an abrasive etching operation, followed by application of a porcelain enamel cover coat directly to the base metal, and fusing said procelain enamel cover coat to the base metal.

7. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between 1100 F. and 1250 F. long enough to o'xidize from about one to. roar grams base metal per square foot of surface andithereafter etching with phos; phoric acid, followed by application of a porcelain enamel cover coat directly to the ba'semetal, and fusinglsaid porcelain enamel cover coat to the base metal.

8. ln'the' process of preparing ferrousbase' metalfor direct-on procelainenameling inwhich the metal issubjected'toa preliminary cleaning operation, the combina tion of heat-treating the cleaned base metal ata tempera-: ture of between 1000 F. and 1300" F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by a phosphoric acid etch, followed by a rinse, followed bynickel deposition of from 0.05 to0.25 gra'm'per square foot, followed by applying a porcelainenamel cover coat to the base metal.

9. In'the process of preparing ferrousbasemetal fo'r direct-on porcelain enameling'in which themetal is subjected t'oa preliminary cleaning operation, the combination of heat-treating the cleaned bas'e-met-al at a temperature of-between about 1000 F; and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by a sulphuric acid-etch, followed by a rinse, followed by nickel deposition of from 0.05 to 0.25 gram per square foot, followed by applying a porcelain enamel cover coat to the base metal.

10. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by a nitric acid etch, followed by a rinse, followed by nickel deposition of from 0.05 to 0.25 gram per square foot, followed by applying a porcelain enamel cover coat to the base metal.

11. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by a hydrochloric acid etch, followed by a rinse, followed by nickel deposition of from 0.05 to 0.25 gram per square foot, followed by applying a porcelain enamel cover coat to the base metal.

12. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by an abrasive etching operation, followed by a rinse, followed by nickel deposition of from 0.05 to 0.25 gram per square foot, followed by applying a porcelain enamel cover coat to the base metal.

13. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination of heat-treating the cleaned base metal at a temperature of between 1100 F. and 1250" F. long enough to oxidize from about one to four grams base metal per square foot of surface followed by a phosphoric acid etch, followed by a rinse, followed by nickel deposition of from 0.05 to 0.25 gram per square foot, followed by applying a porcelain enamel cover coat to the base metal.

14. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination therewith of steps following said cleaning operation comprising heat treating the ferrous base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams of base metal per square foot of surface, removing the oxidized metal layer coincidently with etching the underlying base metal surface in acid, depositing a layer of nickel on said etched surface at a concentration offrom 0.05 to 0.25 gram per square foot, and thereafter applying a porcelain enamel cover coat directly to the base metal.

15. In the process of preparing ferrous base metal for direct-on porcelain enameling in which the metal is subjected to a preliminary cleaning operation, the combination therewith of steps following said cleaning operation comprising heat treating the ferrous base metal at a temperature of between about 1000 F. and 1300 F. long enough to oxidize from about one to four grams of base metal per square foot of surface, removing the oxidized metal layer coincidently with roughening the underlying base metal surface by an abrasive operation, depositing a layer of nickel on said etched surface at a concentration of from 0.05 to 0.25 gram per square foot, and thereafter applying a porcelain enamel cover coat directly to the base metal.

16. In the process of preparing ferrous base metal for References Cited in the file of this patent UNITED STATES PATENTS 2,099,340 Kautz Nov. 16, 1937 2,206,597 Canfield et al. July 2, 1940 2,532,283 Brenner et al. Dec. 5, 1950 2,581,310 Sweo Jan. 1, 1952 2,812,270 Alexander Nov. 5, 1957 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2,961,337 November 22 1960 Eugene E, Bryant et al., I

It is hereby certified'that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read escorrected below. I

In the drawing, the description of the nickel deposition bath, for "Na SO -6H O" read NiSO -6H 0 in the printed specification, column 4, line 18, for "(Na SO 6H O)" read (NiS0 -6H O) Signed and sealed this 20th day of June 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

1. IN THE PROCESS OF PREPARING FERROUS BASE METAL FOR DIRECT-ON PORCELAIN ENAMELING IN WHICH THE METAL IS SUBJECTED TO A PRELIMINARY CLEANING OPERATION, THE STEPS FOLLOWING SAID CLEANING OPERATION OF HEAT-TREATING THE FERROUS BASE METAL AT A TEMPERATURE OF BETWEEN ABOUT 1000* F. AND 1300* F. LONG ENOUGH TO OXIDIZE FROM ABOUT ONE TO FOUR GRAMS BASE METAL PER SQUARE FOOT OF SURFACE, AND THEREAFTER APPLYING A PORCELAIN ENAMEL COVER COAT DIRECTLY TO THE BASE METAL. 